Method of producing a weldable and ageable aluminium alloy of great strength and product

ABSTRACT

The invention provides a weldable and heat-treatable aluminium alloy which comprises from 4.5 to 5.8% zinc, from 1.0 to 1.8% magnesium, from 0.10 to 0.30% zirconium, from 0 to 0.30% iron, from 0 to 0.15% silicon, from 0 to 0.25% manganese, less than 0.05% of each of other elements, the sum of these other elements not exceeding 0.15%, and the balance aluminium, which alloy comprises metastable precipitates of zirconium aluminide uniformly distributed in a number greater than 1010 per mm3, and of a particle size less than 2,000A.

United States Patent 91 Aarflot et al.

[ Dec. 3, 1974 METHOD OF PRODUCING A WELDABLE AND AGEABLE ALUMINIUM ALLOY OF GREAT STRENGTH AND PRODUCT [75] Inventors: Aksel Ola Aarflot; Fred Rudi, both of Sunndalsora; Gunnar Birger Stirensen, Asker; Bjarne Alvsaker, Holmestrand; Otto Berg, Oslo, all of Norway [73] Assignee: A/S Ardal og Sunndal Verk, Oslo,

Norway [22] Filed: Mar. 5, 1973 21 App]. No.2 338,183

[30] Foreign Application Priority Data Mar. 10, 1972 Norway 778/72 [52] US. Cl 148/3, 75/146, 148/325, 148/159 [51] Int. Cl. C22f 1/04 [58] Field of Search 75/146, 141, 138; 148/32, 148/325, 3, 159

Primary Examiner-R. Dean Attorney, Agent, or Firm-Wenderoth, Lind & Ponack 5 7] ABSTRACT The invention provides a weldable and heat-treatable aluminium alloy which comprises from 4.5 to 5.8% zinc, from 1.0 to 1.8% magnesium, from 0.10 to 0.30% zirconium, from 0 to 0.30% iron, from 0 to 0.15% silicon, from 0 to 0.25% manganese, less than 0.05% of each of other elements, the sum of these other elements not exceeding 0.15%, and the balance aluminium, which alloy comprises metastable precipitates of zirconium aluminide uniformly distributed in a number greater than 10 per mm, and of a particle size less than 2,000A.

4 Claims, No Drawings METHOD OF PRODUCING A WELDABLE AND AGEABLE ALUMINIUM ALLOY OF GREAT STRENGTH AND PRODUCT rally aged after welding (AA 7005 basic material with AA 5180 welding wire). v I

Aluminium alloys are also used in equipment for transporting condensed gases at temperatures down to This invention relates to a weldable and ageable alu- 5 270C. The alloys mainly used for this purpose consist minium alloy which is characterized by a fine-grained cast structure and uniform sub-microscopic dispersion of zirconium aluminide precipitations which is suitable for the manufacture of bumpers and other equipment for private cars and commercial vehicles. Further it is well suited for building trailers, buses and railway rolling stock, and also for more general constructions such as masts, pylons and bridges. It is further well suited for making equipment for producing, storing and transporting condensed gases at temperature ranges down to 270C.

The alloy of the invention consists of from 4.5 to 5.8% zinc, from 1.0 to 1.8% magnesium, from 0.10 to 0.30% zirconium, from 0 to 0.30% iron, from 0 to 0.15% silicon, from 0 to 0.25% manganese, less than 0.05% of each of other elements, the sum of these other elements not exceeding 0.15%, and-the balance aluminium.

Aluminium/zinc/magnesium alloys exist today with a number of variations in alloy composition. The weldable aluminium/zinc/magnesium alloys contain from 3.5 to 5.0% zinc and from 0.5 to 2.0% magnesium. In several alloys developed since 1960 there is used, in addition to or instead of chromium, zirconium as a grainrefining and recrystallization-inhibiting element. The zirconium content in, e.g., AA 7005 is from 0.06 to 0.20% and in Unidur-l00 from 0 to 0.20%.

The characteristic feature of the alloy according to the invention is that it contains metastable precipitates of zirconium aluminide uniformly dispersed in a number greater than per mm", and with a particle size of less than 2000A.

The invention also relates to a method for producing the above-mentioned alloy. The method is characterized in that the zirconium is added to the melt, which melt consists of the remaining components of the alloy, in the form of a hardener or master alloy which essentially consists of aluminium and zirconium, at a temperature in excess of 720760C depending on the zirconium content, and in such quantities that the finished melt will contain 0.100.30% zirconium. The melt is held at this temperature for at least 30 minutes while argon or some other inert gas is bubbled therethrough, the melt is cooled to 7l0-720C by still bubbling the same inert gas therethrough, and, the melt is then held at this temperature for about 10 minutes without the addition of gas, whereupon the melt is cast and homogenized in order to obtain metastable precipitates of zirconium aluminide.

The use of aluminium fabricating alloys for motor vehicle components is particularly limited to products such as window frames and decorative trims, whilst aluminium alloys have had little or no use in components intended to withstand major loads such as, e.g., bumpers.

On the other hand, aluminium/zinc/magnesium alloys have to a larger extent been used for building transport material such as railway trucks, trailers and dumpers etc. The alloys which are now being used to an ever increasing degree are weldable and have a yield strength of about 19 kp/mm when the material is natuof a1uminium/magnesium/manganese comprising from 3 to 5% magnesium and from 0 to 1% manganese, mainly NS 17215 (AA 5083). This type of alloy has, after welding, a yield strength of 12 kp/mm and is not heat treatable. Recently, it has been proposed to use an aluminium/zinc/magnesium alloy containing 4% zinc, 2,8% magnesium, 0.25% manganese and 0.20% chromium (AA 7039).

Alloys according to the invention can, in addition to the purposes already mentioned, also be used for manufacturing highway hardware, tools, masts, pylons and cranes, machinery and building structures, and sports equipment.

The purpose of adding small quantities of zirconium is to achieve:

a. a fine-grained cast structure.

b. Stabilization through a high sub-grain concentration.

c. Higher mechanical strength.

d. Reduced sensitivity to heat-cracking during welding.

It is known that zirconium has a positive effect on the above-mentioned properties and that higher recrystallization resistance in particular improves the stress corrosion properties. At the melting point, the solubility of zirconium in aluminium is recorded as 0.11% in the molten phase and 0.24% in the solid phase. The solubility decreases with falling temperature to 0.05% at 500C. As previously mentioned, zirconium concentrations are used in the range of from 0 to 0.20%.

It has been found that the concentration of zirconium is critical for achieving several of the above mentioned properties. Among other things, a zirconium content of 0.05% will not be sufficient to provide an optimum fine-grained cast structure. Furthermore, the resistance to recrystallization will decrease and the sub-grain boundaries will be less stable with decreasing concentration of zirconium in that fewer zirconium aluminide percipitations are formed.

Alloys which contain chromium and also iron in quantities exceeding 0.20%, are clearly prone to exfoliation corrosion in the naturally aged state. The chromium content of the alloy according to the present invention shall not exceed 0.05%. This provides a more uniform dispersion of zirconium aluminides, improved resistance to corrosion and less sensitivity to quenchmg.

One purpose of the invention is, through suitable treatment of the melt and the casting procedure, to ensure a fine-grained cast structure and a sufficient supersaturation of zirconium, so that with subsequent heat treatment in the temperature range from 450C to 560C, depending upon the zirconium content, finely dispersed metastable precipitates of zirconium aluminides will be formed. These precipitations stabilize the grain and sub-grain boundaries and contribute in improving a number of properties.

Another purpose of the invention is to produce a heat-treatable aluminium alloy which is ductile and may be readily formed and which satisfies the properties required foruse in bumpers and similar motor vehicle components where the demands on mechanical strength are particularly high.

A further purpose of the invention is to provide a weldable and heat-treatable aluminium alloy of higher mechanical strength after welding than that of other aluminium alloys, with the aim of producing load-bearing structures in transport material and more generally in masts, pylons, and smaller bridge constructions.

Yet a further purpose of the invention is to provide a weldable and heat-treatable aluminium alloy of higher mechanical strength than that of other aluminium alloys, particularly after welding, for use in equipment for producing, storing and transporting condensed gases at temperatures down to 270C.

The above-mentioned advantages are achieved with the alloy according to the invention.

The following example illustrates the production of the alloy according to the invention.

EXAMPLE in order to achieve a satisfactory grain-refining and sufficient supersaturation with zirconium, the treatment of the melt and the casting must be performed as follows:

The alloying elements, zinc and magnesium, are added to the aluminium melt in accordance with normal practice. The melt is heated to a temperature in excess of 720C, and zirconium is added in a quantity of 0.10 to 0.30% in the form of a hardener (master alloy) which substantially consists of aluminium and zirconium. in order to ensure the complete dissolution of the zirconium in the melt, the latter is held at a temperature of at least 720C for at least 30 minutes. The temperature is then reduced to about 7l0720C while argon is caused to bubble through the melt in order to give the bath a thorough stirring. Instead of argon, some other inert gas can be used.

After this the melt is allowed to stand for 10 minutes without gas passing through it to allow coarse zirconium particles to settle. For continuous casting, the temperature of the melt in the furnace should be about 720C and in the distributing basin, about 700C.

After casting and cooling the material must be heattreated at a temperature between, 450 and 560C as previously stated, e.g., during 8 hours at 480C or 5 hours at 550C, in order to ensure a uniform dispersion of metastable precipitates of zirconium aluminide. This can be carried out simultaneously with homogenizing, which may be carried out at a temperature between 400 and 550C.

The above-mentioned alloy will, immediately after the cooling of extruded shapes or after separate solution heat-treatment, be sufficiently ductile for the roll forming of, e.g., bumpers for private cars and commercial vehicles. On subsequent storage at room temperature the alloy ages naturally achieving a mechanical strength which satisfies the specifications required of motor vehicle bumpers for sale on the American market.

The alloy of the invention is well suited for loadbearing structures in transport material and for other general structural purposes. The basic alloy, in the naturally aged state, has a tensile strength of from 38 lcp/mm to 40 kp/mm, and a yield strength of from 25 ltp/mm to'30 kp/mm. 1n the artificially aged state, the tensile strength is from 40 kp/rnm to 58 kp/mm and the yield strength is from 38 kp/mm to 55 kplmm After welding, the tensile strength in the naturally aged state is within the range from 30 kp/mm to 35 kplmm and the yield strength from 25 kp/mm to 30 kplmm After welding, the tensile strength in the artificially aged state is within the range 33 kp/mm to 40 kp/mm and the yield strength from 31 kp/mm to 40 kp/mm? The alloy according to the invention is, as mentioned above, well suited for use in making equipment for the production, storing, and transportation of condensed gases at temperatures down to 270C. With falling temperatures, its mechanical strength increases without its ductility deteriorating.

The advantages achieved through the alloy of the invention are:

a. By adding from 0.10 to 0.30% zirconium to the melt, a sufficient supersaturation with zirconium is achieved, so that the cast material acquires a finegrained structure which improves the extrudability of the alloy. The extruding pressure can be reduced and/or the extruding speed can be increased. The finely dispersed zirconium aluminide precipitations stabilize the sub-grain structure and help to increase the resistance against recrystallization. In this manner, higher mechanical strength is achieved, as isimproved ductility and less likelihood of heat-cracking during welding.

b. 1n the solution heat-treated state, the alloy is sufficiently ductile for roll forming, and its high mechanical strength is recovered after natural artificial age'ing.

c. Good welding properties of the alloy and in particular good mechanical properties can berecovered after welding through subsequent natural or artificial ageing. Artificial ageing; results in a yield strength of over 31 kp/mm, which permits the building of structures with a much lower weight than when using other weldable aluminium alloys.

d. Considerable savings are achieved in material and weight in welded structures and equipment which are to be used at temperatures down to 270C.

What is claimed is:

I. A weldable, heat-treatable aluminium alloy consisting essentially of from 4.5 to 5.8% zinc, from 1.0 to 1.8% magnesium, from 0.10 to 0.30% zirconium, from 0 to 0.30% iron, from 0 to 0.15% silicon, from 0 to 0.25% manganese, less than 0.05% each of other elements, the sum of these other elements not exceeding 0.15%, and the balance being aluminium, which alloy comprises metastable precipitates of zirconium aluminide having a particle size less than 2000 A uniformly distributed in the alloy in an amount greater than 10" per mm.

2. A method for producing an alloy consisting essentially of from 4.5 to 5.8% zinc, from 1.0 to 1.8% magnesium, from 0.10 to 0.03% zirconium, from O to 0.3% iron, from 0 to 0.15% silicon, from 0 to 0.25% manganese, less than 0.05% each of other elements, the sum of these other elements not exceeding 0.15%, and the balance being aluminium, according to claim 1, which comprises (1) adding zirconium, in the form of a hardener or master alloy which consists essentially of aluminium and zirconium, to a melt containing the remaining components of the alloy at a temperature in excess of 720760C depending on the amount of zir-.

melt and (6) homogenizing the cast melt to obtain metastable precipitates of zirconium aluminide therein.

3. The method according to claim 2, wherein the homogenizing is carried out at a temperature between 400 and 550C.

4. 'The method according to claim 2, wherein the inert gas is argon.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,852,122 Dated December 3, 1974 Inventor s AKSEL OLA AARFLOT ET AL.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby correoted as shown below:

-In claim 2, line 3, ohange "0.03%" to -0.30%-.

Signed and Sealed this twenty-fifth Day of November 1975 {SEALI I A ttest:

Rum c. M'ASON c. MARSHALL DANN .lrlesting ()ffiver. (unzmissimurj ufPatenrs and Trademarks 

1. A WELDABLE, HEAT-TREABLE ALUMINIUM ALLOY CONSISTING ESSENTIALLY OF FROM 4.5 TO 5.8% ZINC, FROM 1.0 TO 1.8% MAGNESIUM, FROM 0.10 TO 0.30% ZIRCONIUM, FROM 0 TO 0.30% IRON, FROM 0 TO 0.15% SILICON, FROM 0 TO 0.25% MANGANESE, LESS THAN 0.05% EACH OF THER ELEMENTS, THE SUM OF THESE OTHER ELEMENT NOT EXCEEDING 0.15%, AND THE BALANCE BEING ALUMINUM, WHICH ALLOY COMPRISES METASTABLE PRECIPITATES OF ZIRCONIUM ALUMINIDE HAVING A PARTICLE SIZE LESS THAN 2000 A UNIFORMLY DISTRIBUTED IN THE ALLOY IN AN AMOUNT GREATER THAN 10**-10 PER MM3
 2. A method for producing an alloy consisting essentially of from 4.5 to 5.8% zinc, from 1.0 to 1.8% magnesium, from 0.10 to 0.03% zirconium, from 0 to 0.3% iron, from 0 to 0.15% silicon, from 0 to 0.25% manganese, less than 0.05% each of other elements, the sum of these other elements not exceeding 0.15%, and the balance being aluminium, according to claim 1, which comprises (1) adding zirconium, in the form of a hardener or master alloy which consists essentially of aluminium and zirconium, to a melt containing the remaining components of the alloy at a temperature in excess of 720*-760*C depending on the amount of zirconium, and in an amount such that the finished melt will contain 0.10-0.30% zirconium, (2) maintaining the melt at said temperature for at least 30 minutes while bubbling an inert gas through the melt, (3) cooling the melt to 710*-720*C while bubbling the inert gas through the melt, (4) maintaining the melt at said reduced temperature for about 10 minutes without bubbling the inert gas through the melt, (5) casting the melt and (6) homogenizing the cast melt to obtain metastable precipitates of zirconium aluminide therein.
 3. The method according to claim 2, wherein the homogenizing is carried out at a temperature between 400 * and 550*C.
 4. The method according to claim 2, wherein the inert gas is argon. 